Stephens shocks Williams to reach semi-final

American teenager Sloane Stephens ended Serena Williams' bid for a historic calendar-year Grand Slam when she sent the injury-hampered favourite crashing out of the Australian Open Wednesday.

As Williams, who was troubled by ankle problems earlier in the tournament, required treatment for back spasms and smashed her racquet as the match slipped away, Stephens held her nerve to win 3-6, 7-5, 6-4 in two hours, 17 minutes.

"This is so crazy," said Stephens, who used to have Williams' poster on her bedroom wall.

Back trouble

At 3-4, 0-30, Williams grimaced after hitting a backhand winner. Close to the net, she had to pull up quickly to avoid making contact with the net and losing the point.

"I even screamed on the court," she said.

"I was like, aagh - I totally locked up after that. It was a little painful. It was hard to rotate to the backhand. I'm already on anti-inflammatories for my ankle, so you can't do too much more."

She began hobbling between points, lost strength on her serve and Stephens broke for 5-3. Williams attempted to keep rallies short by hitting winners, and Stephens had an attack of nerves to lose serve when trying to level it at a set apiece.

Williams called for the trainer at 4-5, complaining of back spasms. Next game, she could barely serve but her groundstrokes remained potent.

She held serve to love as Stephens lost composure, but the teenager staved off a break points in the following game to lead 6-5, and won her second set point to take the match into a decider.

Games went on serve until Williams broke Stephens for a 4-3 lead. Williams' movement had improved to a significant degree, but Stephens broke back amid excruciating tension.

A final break of serve gave the teenager the biggest win of her career.